1. Field of the Invention
The present invention relates to a thermal spray material having excellent corrosion resistance and heat resistance, and a thermal sprayed member formed by spraying the thermal spray material on the surface of a metallic member.
2. Description of the Prior Art
Recently, very excellent high-temperature properties are required for a heat resistant/corrosion resistant member such as a gas turbine blade. The efficiency of the operation of the gas turbine increases in accordance with the rise in operation temperature. Under the situation, the rise in operation temperature is always demanded. To meet the demand, there has been developed a heat resistant, corrosion resistant and creep resistant ceramic material, such as SiC or Si.sub.3 N.sub.4. This material, however, has unsatisfactory impact strength and thermal shock resistance, and is unreliable as material of parts of the gas turbine. At present, therefore, the gas turbine is formed of a material consisting mainly of a Ni-base alloy or a Co-base alloy. The working temperature of this type of heat resistant material is limited to 1000.degree. C. When this heat resistant material is used as that of parts of the gas turbine, the parts are cooled in use, the parts are thermal barriered, or other methods are employed. According to one method of barriering the parts from heat, the metal substrate of the parts is coated with ceramic layers, thereby preventing the rise in temperature of the substrate. In this process, ceramic particles having a low thermal conductivity, a high thermal shock resistance and a high radiation coefficient have conventionally been employed as a thermal spray material. More specifically, the employed ceramic material is, for example, yttria stabilised zirconia (YSZ). Even if the layer of thermal spray material is sprayed on the part of the gas turbine, it may be peeled off the substrate owing to the heat cycle in which rapid heating and rapid cooling is alternately performed. As a result, the thermal barrier function of the sprayed part is lost. In addition, this type of spray material is expensive, resulting in a rise in manufacturing cost.
In general, the thermal sprayed layer tends to be peeled off the substrate owing to the heat cycle in which rapid heating and rapid cooling is repeated. It is thus desirable that the thermal spray material has not only a low thermal conductivity, but also a thermal expansion coefficient close to that of the substrate. The materials meeting these requirements have been widely developed. In addition, there are proposed a technique of providing an intermediate layer formed of a mixture of metal and ceramics, on the metal substrate (Published Unexamined Japanese Patent Application No. 55-113880), a technique of subjecting a ceramic layer, which is thermal sprayed on the metal, to a heat treatment at high temperature and for a long time, thereby producing micro-cracks in the ceramic layer (Published Unexamined Japanese Patent Application No. 56-54905), and a technique of rapidly cooling the formed ceramic layer thereby producing micro-cracks therein (Published Unexamined Japanese Patent Application No. 58-87273).
However, even if the intermediate layer of metal and ceramics is provided, the corrosion resistance and thermal shock resistance is not enhanced satisfactorily. In addition, even if micro-cracks are produced in the ceramic layer by heating or rapid cooling, the corrosion resistance and thermal shock resistance is not enhanced satisfactorily, while the number of manufacturing process is increased.